Aqueous alkaline solutions are easy to handle, and therefore it have been increasingly employed as etching agents for silicon wafers instead of a hard-to-handle mixed acid which contains hydrofluoric acid, nitric acid and acetic acid. The aqueous alkaline solutions, however, contain trace amounts of metals such as iron, nickel, molybdenum and/or copper. Such metals are derived from raw materials for the aqueous alkaline solutions or contaminated during production processes. These impurities containing metals such as iron, nickel, molybdenum and/or copper penetrate a silicon wafer during etching, remain therein after etching and thus cause change in electrical insulating properties of the silicon wafer. Accordingly, such aqueous alkaline solutions containing a certain amount of these metallic impurities are substantially difficult to use as etching agents practically.
In order to use in etching of silicon wafers, the content of each of metallic impurities which are contained in trace amounts in the aqueous alkaline solutions should be reduced to 200 ppb or less, more preferably 100 ppb or less. More specifically, it has been demanded to reduce an iron content to 100 ppb or less, more preferably 10 ppb or less; a molybdenum content to 100 ppb or less, more preferably 10 ppb or less; and a nickel content to 10 ppb or less, more preferably 1 ppb or less.
Certain methods for purifying an aqueous alkaline solution have been known to use activated carbon. However, no method is known which reduces metallic impurities to such concentrations as to be used for the aqueous alkaline solutions etching of silicon wafers. The patent document 1 as described below discloses a method for purifying an aqueous sodium hydroxide solution by using an activated carbon. In the method, iron contained in the aqueous sodium hydroxide solution is removed with the use of activated carbon, thereby to avoid problems in production of sodium hypochlorite. This document, however, merely discloses that iron contained as impurity can be removed to a content of 2 ppm in terms of Fe2O3 by allowing the aqueous sodium hydroxide solution to pass through a layer of granular activated carbon.
Patent document 2 described below relating to a method for removing nickel discloses that the nickel content can be reduced to the order of about 10 ppb by allowing an aqueous potassium hydroxide solution to pass through a filtrating device precoated with activated carbon derived from coconut shell. This document, however, merely discloses a specific example in which the nickel content is reduced to about 50 ppb. In addition, the used activated carbon is granular activated carbon prepared from coconut shell and is apt to become fine powder. This often invites clogging of the filtrating device and/or scattering of dust and thereby requires excessive facilities such as dustproof devices in practical use. In addition, the activated carbon is difficult to reactivate, which obstructs reuse of the resources.
Furthermore, techniques wherein an activated carbon is used to remove copper have not been known yet.
The patent document 3 described below discloses another method for purifying an aqueous sodium hydroxide solution by removing metallic impurities therefrom, in which the aqueous sodium hydroxide solution is purified by electrolysis with a cation-exchange membrane. This document mentions that this method can reduce the concentration of metallic impurities in the aqueous sodium hydroxide solution to 10 ppb or less. This method, however, is low in efficiency, since metallic impurities are removed while increasing the concentration of the aqueous sodium hydroxide solution by further electrolyzing an aqueous sodium hydroxide solution once prepared by electrolysis of sodium chloride.    Patent document 1: Japanese Patent Application, First Publication No. Sho 52-52898    Patent document 2: Japanese Patent Application, First Publication No. 2000-203828    Patent document 3: Japanese Patent Application, First Publication No. 2002-317285